The reason is the networking model used for RTS, which is generally Deterministic Lockstep. It requires that all participants have all information at all times, and reconnecting is difficult using that system. This is/was used because it is comparatively low bandwidth, particularly when large numbers of units are present.
Benefits: Fast, cheap (In terms of data)
Negatives: Reconnecting means you need to simulate the entire game up to that point, and cheating is easier/more common, as each client gets data for everything in the game.
Also: There's always some input delay, which gets worse as latency increases. This is why it can take so long for your units to actually respond to your commands.
With the advances in hardware/bandwidth; It's perfectly possible to use a client/server model, or some sort of hybrid. This would
Planetary Annihilation is an example of an RTS that uses a client/server/predictive model, and it also has a reconnection feature.
As before: A reconnect feature is possible in CoH2, but it would be less straightforward than in, say, an FPS, and it would mean you'd basically need to watch a replay of what happened up to the point you reconnect.
I know what you said, but what i mean is that ALL AAA titles seems to not deem it worthy for automatch purposes. For Planetary Annihilation, can you reconnect AFTER dropping from the game or is it like COH or other titles on which you have a certain leeway before dropping totally and not been able to reconnect.
There are other titles that allow reconnection but not for automatch. Like AOE/SC2.
I would say this: if the current team is unable to fix spectator mode and we don't even have a way to rewind during replays, don't expect to be an advancement in technology that allows for a decent reconnection system on RTS.